, 2000). STs sharing identity at the majority of these loci are grouped into clonal complexes (CCs) encompassing related lineages of MRSA (Enright et al., 2002). Another highly discriminatory approach that can identify genomic rearrangements and insertions/deletions is pulsed-field gel electrophoresis (PFGE) whereby SmaI digested chromosomal DNA is separated

and similarities in banding patterns reflect relatedness among lineages (Bannerman et al., 1995; McDougal et al., 2003). Selleck Y 27632 This allows for the classification of S. aureus strains into the now familiar PFGE types USA100-1200. Employing these epidemiological approaches, researchers appreciated that most MRSA disease worldwide (nearly 70% of reported infections) was caused by five major CCs: CC5, CC8, CC22, CC30, and CC45 (McDougal et al., 2003; Robinson & Enright, 2003) (Fig. 1). CC5 includes clones belonging to the USA100 PFGE type (e.g. SCCmec-II New York/Japan clone), the most common source of US hospital-acquired MRSA as well as USA800 (SCCmec-IV Pediatric clone). CC8 includes the archaic, or original MRSA clones as well as the

related Iberian clone, the SCCmec-III Brazilian/Hungarian clone, and the SCCmec-IV USA500 clones. CC22 includes the EMRSA-15 clones that dominated hospital infections in the UK during the 1990s along with strains from CC30 encompassing EMRSA-16 as well as the USA200 PFGE type. Finally, CC45 consists of clones belonging to USA600 PFGE type (e.g. Berlin Antiinfection Compound Library chemical structure clone) that caused widespread MRSA hospital infections in PtdIns(3,4)P2 northern Europe. In essence, after 30 years of investigation, the scientific community began to understand the population

structure of the MRSA clones responsible for the majority of hospital-acquired disease. The source of high virulence potential inherent to these five CCs was never fully appreciated before everything we knew about MRSA epidemiology changed at the turn of the century. Initially reported in 1993, patients without any contact with healthcare settings contracted invasive MRSA infections in Kimberly Australia, a region in the northern part of Western Australia (Udo et al., 1993). It was later discovered that simultaneously, strains related to these ‘community-acquired’ MRSA (CA-MRSA) clones were causing serious and fatal respiratory infections in Chicago, again in patients without direct contact with hospital environments (Center for Disease Control & Prevention, 1999). Prior to these reports, MRSA infections were exclusively associated with healthcare settings. These new clones belong to CC1 (USA400 PFGE type), a CC unrelated to the five traditional hospital-associated MRSA (HA-MRSA) complexes (Center for Disease Control & Prevention, 1999).